In the field of fish farming, the maintenance of water takes more precedence. The water quality determined by the organic content and dissolved oxygen has a great influence on keeping fish. The organic content in the aquatic water comes from the excretions of the fish and the remained fish food, most of which is remained in the form of particles at the bottom of the fishpond.
In the prior art, the mechanism of the filtration system for fish-tank is that internal filtration materials blocks the particles pollutant to purify the water. As time goes on lots of pollutant will accumulate on the filtration materials. This will reduce the water flow rate and the filtration effect, so washing the filtration materials is a time-consuming work. One preferred example for the industrialized fish farming is that a sedimentation tank or a rotating separation module is used for separating and discharging the particles pollutant, and then a biological filtration treatment is applied to purify the water. However, these schemes remain many disadvantages, for instance, the separation effect for the sedimentation tank or a rotating separation module is very low because of the huge force applied to the particles by the strong water flow. To reach an ideal effect, the sedimentation tank or a rotating separation module should be enlarged to a large scale. In this condition, the pollutant discharging and the back flushing need a high quantity of water. So a powerful filtration system and a large quantity of fresh water are needed to maintain the water quality with a high construction cost and a low use ratio.